1. Field of the Invention
The present invention relates to a hydraulic shock absorber for damping vibration.
2. Description of the Related Arts
A hydraulic shock absorber is used, for example, for damping vibrations of a vehicle body of a road vehicle. In the hydraulic shock absorber, a cylinder is partitioned into first and second fluid chambers with a piston, and a fluid flows between the first and second fluid chambers synchronized with vibration when the vehicle body is vibrated. The vibration is damped by resistance generated when the fluid flows between the first and second fluid chambers. Japanese Patent Application Publication No. 2000-110881 discloses a hydraulic shock absorber in which a though-hole is bored through a piston or a passage is bored through the piston rod in its axial direction to allow the fluid to flow between the first and second fluid chambers.
FIG. 20 is a graph for showing a relationship between the damping force and the piston speed in the conventional hydraulic shock absorber.
Recently, a mechanism for adjusting the damping force characteristic of the hydraulic shock absorber has been required to improve the ride quality of a vehicle. Japanese Patent Application Publication No. 9-291961 discloses a conventional hydraulic shock absorber for a vehicle having such a damping force characteristic that a damping force is largely varied in a fine low speed zone of the piston speed of a piston which moves up and down in a cylinder and the damping force is slightly varied in the low and medium speed zone of the piston speed as shown in FIG. 20.
In this hydraulic shock absorber, a leaf valve does not compress a spring but working oil bends the outer peripheral part of the leaf valve and flows out when the hydraulic shock absorber extends or contracts at the fine low speed, whereby its damping force is generated according to the opening degree of the leaf valve. Thus, the first slope of the damping force is large in the fine low speed zone as shown in FIG. 20.
In a speed zone of the low and medium speed zone or higher, the leaf valve compresses an energizing unit (spring) and the outer peripheral part of the leaf valve is also bent, whereby the leaf valve is fully opened, adjusting the damping force generated. Therefore, the inclination of the damping force in a speed zone of the low and medium speed zone or higher is smaller than the first slope of the damping force in the fine low speed as shown in FIG. 20, which means that the increasing rate of the damping force is decreased in the damping force characteristic of the hydraulic shock absorber.
Moreover, as disclosed in Japanese Patent No. 3479647, a hydraulic shock absorber (vibration damper) is conventionally known which includes a first piston fixed to an end of a piston rod and a second piston which is slidably provided to the piston rod radially inner than the first piston.
In the hydraulic shock absorber disclosed in Japanese Patent No. 3479647, the first piston and the second piston both includes a valve, and when the two valves are opened, a flow passage between fluid chambers in a cylinder is communicated so that the fluid flows between the fluid chambers, whereby the damping force is adjusted. In short, the hydraulic shock absorber is allowed to change its damping force in two-step by the movement amount of the piston rod (amplitude). The second piston is supported by springs fixed to the piston rod at its both ends in the axial direction.
If the movement amount of the piston rod is small, only the valve of the first piston is opened to generate a small damping force, the valve of the second piston is not opened and the spring is compressed in the hydraulic shock absorber. More specifically, the damping force generated in the entire hydraulic shock absorber in the case where the movement amount of the piston rod is small is determined by the damping force generated by the first piston and reaction force of the spring. At this time, the damping force is so small that the first piston hardly moves relative to the cylinder.
If the movement amount of the piston rod is large, the damping force is large in the hydraulic shock absorber since the damping force is generated by both of the first piston fixed to the piston rod and the second piston. At this time, if the floating piston slams into the spring seat, a valve disk is opened, and the piston becomes to move relative to the cylinder.
The reason for providing a communication passage through which a fluid flows between first and second fluid chambers to each of the piston and the piston rod as disclosed in Japanese Patent Application Publication No. 2000-110881 is to realize a variable attenuating force hydraulic shock absorber for changing a damping force for damping vibration according to a vibration status. The communication passage provided to the piston can be readily formed by forming a through-hole in the piston. On the other hand, the manufacturing work of the communication passage provided to the piston rod is complicated and its manufacturing cost is also high because the piston rod has to be formed hollow to form a flow passage extending in the axial direction, and radial direction paths have to be also provided which extend from the outer circumferential surface of the piston rod to the flow passage extending in the axial direction. As described above, at least two passages are desired as a communication passage. Thus, at least one communication passage is desired other than the communication passage formed by the through-hole provided in the piston. The manufacturing work of the conventional communication passage provided in the piston rod is so complicated that a communication passage which can replace the conventional communication passage has been desired.
FIGS. 21A and 21B are graphs showing damping force characteristics of the hydraulic shock absorber disclosed in Japanese Patent Application Publication No. 9-291961. FIG. 21A shows a relationship between the piston speed and the damping force. FIG. 21B shows a relationship between the damping force and time.
As shown in FIG. 21A, the hydraulic shock absorber disclosed in Japanese Patent Application Publication No. 9-291961 prevents a deterioration of the ride quality by making its damping force to be smaller when the piston speed is high, compared to a hydraulic shock absorber in which the spring is not provided to the piston.
In the hydraulic shock absorber disclosed in Japanese Patent Application Publication No. 9-291961, however, the ride quality of a vehicle is deteriorated when a vehicle runs over large bumps on a road which is not maintained because the damping force is sharply increased in proportion to the piston speed as shown in FIG. 21B.
The technique of Japanese Patent Application Publication No. 9-291961 would give “rigid-feeling” which is attributed to the magnitude of the damping force because the damping force is increased sharply (i.e. a large damping force is generated rapidly when a large input load is applied), and this deteriorates the ride quality of a vehicle.
The hydraulic shock absorber disclosed in Japanese Patent No. 3479647 would give “step feeling” because the damping force characteristic is switched by the opening or closing of the valves provided in the second piston and resistance is varied stepwise at the time when the damping force characteristic is switched. This deteriorates the ride quality of a vehicle.
Since the hydraulic shock absorber includes a second piston in addition to the first piston which is fixed to the piston rod, the hydraulic shock absorber becomes long in the axial direction of the rod, which disadvantageously makes the entire size of the hydraulic shock absorber larger. Further, the hydraulic shock absorber has disadvantages that the number of components and assembly man-hours are large so that its cost is increased and friction resistance of the piston when the piston rod slides is increased because the first piston and the second piston are provided in the cylinder.